Gel is considered as a semi-solid rich in solvent. It consists of 5-10% solid material in the form of a three-dimensional cross-linked network filled with the solvent. In general terminology, alcohol filled gel network is known as alcogel. Whereas an aerogel is a specific class of light weight porous solid material having air or gas in its pores instead of liquid, which is made possible using supercritical drying of alcogel. In the similar way, phosphor aerogel is a special class of luminescent material having narrow particle size distribution with remarkable luminescent properties useful for several strategic applications such as display, lighting, insulating, catalytic and sensing devices.
Synthesis of phosphor aerogel with controlled particle sizes, monolithic, crack free and near uniform luminescent properties is a challenging and tough task. No one has till date reported the preparation process for the same. The formation of cracks takes place during gel settling time and difficult to form monolithic gel.
In the last few decades several processes have been claimed worldwide for the synthesis of phosphor and aerogels separately for various applications including lighting, display and insulated devices.
Reference may be made to Kistler, S. S., 1931 Nature, Vol. 127, page 741. Discovery of Aerogels: Coherent expanded aerogels and jellies, wherein it is suggested that solvent in a gel could be replaced by much rarer medium (i.e. gas) with little or no shrinkage. For the synthesis of gas filled gels the solvent from gels was displaced by other solvents of low critical point. In the process, the gel (containing low critical temperature and pressure conditions of the solvent) was placed in an autoclave with an excess of solvent, and temperature was raised above its critical point to get gas filled gel (aerogel). The processes of the prior art are: (i) enormously time consuming, (ii) involve multiple steps, (iii) involve change of multiple solvents from higher to lower critical conditions, (iv) have less probability of resulting into monolithic (single piece) gels without cracks, etc. However, the present invention overcomes all the above-mentioned drawbacks.
Reference may be made to Kistler, S. S., 1932. Journal of Physical Chemistry, Vol. 36, pages52 -64. First Synthesis of Organic and metal oxide aerogels: Coherent expanded aerogels, wherein it is reported that aerogels of SiO2, Al2O3, WO3, Fe2O3, SnO2, Ni tartrate, cellulose, nitrocellulose, gelatin, agar and egg albumin can be made following the procedure cited in his 1931 reference. The drawbacks associated with the said prior art are similar to the above reference (Kistler, S. S., 1931 Nature. Vol. 127, page 741.) as the preparation procedure is same. However, the present invention overcomes all the above-mentioned drawbacks.
Reference may be made to U.S. Pat. No. 2,188,007, wherein recited is the process for the synthesis of inorganic aerogel compositions, which provides the synthesis (economical) and advantages of aerogels, as depicted by the earlier two papers of the same inventor.
Reference may be made to U.S. Pat. No. 4,873,218, wherein it is recited that the poly-condensation of resorcinol with formaldehyde under alkaline conditions results in the formation of surface functionalized polymer “clusters”. The covalent cross linking of these “clusters” produces gels which when processed under supercritical conditions, produce low density, organic aerogels (density ≤100 mg/cc; cell size ≤0.1 microns). The aerogels are transparent, dark red in color and consist of interconnected colloidal-like particles with diameters of about 100 Å. The said patent deals with the preparation of organic aerogels and upon heating to 600° C.-1200° C. resulted in carbon foam. The aforesaid process includes many tedious steps and suffers from several drawbacks/limitations associated with the said prior art. However, the present invention has few steps and upon heating to similar temperature range results in oxide based aerogels.
A large number of papers have proposed for the synthesis of aerogels and phosphors separately. In general, aerogels are produced using sol-gel process in which the solvent is extracted in order to obtain a porous (>90%) texture of very low density (<100 kg m−3).
Reference may be made to U.S. Pat. No. 5,795,557, in which silica aerogels have been synthesized for high thermal insulation properties by modifying earlier method of Kistler by introducing gazing sector in gels using silicone oils. The drawbacks associated with the said prior art involves (i) the presence of highly viscous silicone oils during preparation, (ii) use of liquid CO2 instead of polar solvents in the autoclave, and (iii) HF has been used as a catalyst during the preparation of gels. However, the present invention does not involve use of any oils, hazardous catalysts or expensive solvents ha during preparation and autoclaving of the gels.
Reference may be made to Kistler, S. S., 1934. Journal of Industrial Engineering and Chemistry, Vol. 26, pages 658-662. Identification of the ultralow thermal conductivity of aerogels: Thermal conductivity of aerogels, wherein the heat conductivity of the aerogels have been measured under variable mechanical and air pressure, and it is suggested that silica aerogels have lowest heat conductivity at atmospheric pressure in comparison with any other insulators. The main drawback associated with the said prior art is the usage of dichlorodifluoromethane (CCl2F2) during the preparation and autoclaving procedures. This chemical is well-known for ozone depletion and hence restricted in many countries. However, the present invention does not involve use of any restricted chemicals that affects environment.
Reference may be made to T. R. N. Kutty: Mater. Res. Bull; 1990, Vol. 25, page 485, wherein it is suggested that sol-gel process produces sub-micron sized particles having the same morphology. To get particles of lower sizes (<micron) in a solid state process, grinding, crushing and milling techniques are preferred. However, in a solid state process it is difficult to control the morphology of the particles and also the efficiency reduces drastically. Moreover, in the present invention no such solid-state processes were used to get the uniform particle size.
Reference may be made to U.S. Pat. No. 2,316,079, wherein it is proposed to prepare water dispersible alumina aerogels that form stable colloidal dispersions or solutions having unique properties. The drawbacks associated with the said prior art are (i) lack of transparency in the visible region, and (ii) water dispersibility. The present invention overcomes this and provideshighly transparent wet gels which are not water dispersible.
Reference may be made to U.S. Pat. No. 5,985,176, wherein recited is a process of synthesis of zinc silicate carried out at low temperature than a normal solid state reaction proposed earlier. The main drawbacks associated with the said prior art are (i) the extended time period (few days) required for formation of wet gels, (ii) followed by drying at ambient temperature and pressures to have xerogels. However, the present invention requires reduced amount of time i.e. few hours for the formation of gels and drying was performed in autoclave under supercritical conditions of the polar solvent.
In short, it may be summarized that all the above processes suggested how to synthesize phosphors and aerogels separately, but no patents and processes deal about how to synthesize phosphor aerogels (intrinsic as well doped) solids having luminescence, high porosity, low density and high thermal insulating properties.
Thus, keeping in view the drawbacks of the hitherto reported prior art, the inventors of the present invention realized that there exists a dire need to provide a process (sol gel followed by annealing) for the bulk preparation of phosphor aerogels of uniform size distribution having luminescence, high porosity, low density and high thermal insulating properties.